Communication device comprising an off-hook detection circuit

ABSTRACT

The subject of the invention is a communications device connected in parallel with at least one item of communications apparatus on a transmission line (L 1 , L 2 ). The device and the apparatus conform to a given template in terms of DC components. The device in accordance with the invention is characterized in that it comprises means ( 18 , R 1 , R 2 , R 3 , D 5 , DZ) setting the operating point (X) in terms of DC component of the said device alone outside regions of the said template where the DC component of the current may be zero, means (R 2 , D 6 , D 7 , R 4, 19, 20 ) of determining the DC component of the current flowing in the said device, means ( 20 ) of detecting a variation in the said DC current component. The invention applies particularly in the field of communication via the switched telephone network.

FIELD OF THE INVENTION

The subject of the invention is a communications device includingdetection of the unhooking of telephone instruments, modems orcommunications apparatus linked to the same telephone line. Theinvention applies particularly to automatic detection of the use of atelephone instrument by a data transmission modem.

BACKGROUND OF THE INVENTION

There is increasing use of apparatus of a type in which a switchedtelephone network (hereinafter “STN”) is connected in parallel with aconventional telephone instrument. Such items of apparatus commonlyemploy a modem connected to the STN line to modulate or demodulate thedata to be sent or to be received. However, apparatus now exists, suchas certain digital television decoders, which uses a modem to establisha connection back to the program provider. In such digital televisiondecoders, it is common practice to establish the connection back to theservice provider at night time so as to avoid any inconvenience to theuser.

The viewer therefore risks finding the telephone line busy when he picksup the handset. This can be inconvenient and even dangerous when anurgent call has to be made.

SUMMARY OF THE INVENTION

The subject of the invention is a communications device connected inparallel with at least one item of communications apparatus on atransmission line, the said device and the said apparatus conforming toa given template in terms of DC components,

the said device being characterized in that it comprises:

means setting the operating point of the said device alone, in terms ofDC component, outside regions of the said template where the DCcomponent of the current may be zero,

means of determining the DC component of the current flowing in the saiddevice,

means of detecting a variation in the said DC current component.

Given that the behaviour, in terms of DC components, of the otherapparatus connected to the transmission line cannot be controlled, theoperating point of the device in accordance with the invention is chosenappropriately in order to be able to obtain a detectable variation inthe DC component of the current in the said device when one of the itemsof apparatus comes into operation.

By choosing the operating point of the device (when it is operatingalone) outside of regions of the template where certain items ofapparatus may (but do not necessarily) have a zero or low DC currentcomponent, a variation of the current in the said device is ensured.

According to one particular embodiment, the said means of setting theoperating point comprise means of generating a characteristic, in termsof DC components, such that it possesses intersections with the possibleload characteristics of the transmission line which are situated inregions of the said template where the DC current component cannot bezero.

The intersection between the load characteristic of the line and thecharacteristic of the device gives the operating point sought.

According to one particular embodiment, the said characteristic in termsof DC components comprises a point of intersection with the 1400 ohmload characteristic, the said point being situated beyond 8.6 V.

According to one particular embodiment, with the said template includinga maximum current limit, the characteristic of the said device includesa maximum slope in the region bounded by the load characteristics andthe said maximum limit, the said characteristic having a point which iscommon with each possible load characteristic.

The characteristic of the device extends throughout the possibleoperating region defined by the different load characteristics, themaximum slope maximizing the variation in current introduced, forexample, upon unhooking a telephone instrument connected to the sameline.

According to one particular embodiment, the said means of detecting avariation in the said DC component comprise a circuit forming thederivative of the signal corresponding to the said defined DC component.

According to one particular embodiment, the said means of detecting avariation in the said DC component comprise means of comparing the saiddefined DC component and the DC component at the operating point of thesaid device alone.

According to one particular embodiment, the said device comprises abridge rectifying the voltage of the line interposed between thetransmission line and an audio signal processing circuit.

According to one particular embodiment, with the positive output of therectifier bridge being connected to the collectors of a first and secondtransistor mounted as an amplifier, the said positive output beinglinked to the base of the first transistor through a first resistor, thesaid amplifier feeds into a second resistor linked to the neutral outputof the said rectifier bridge.

According to one particular embodiment, the said device furthercomprises a capacitor linking the base of the first transistor to theneutral output of the rectifier bridge.

According to one particular embodiment, the said device furthercomprises, in series between the input of the amplifier and the neutraloutput of the rectifier bridge, a third resistor, an on/off means and afirst reference voltage generator, the said on/off means being closedwhen the line voltage exceeds a given value.

According to one particular embodiment, the said on/off means and thesaid voltage source comprise a Zener diode.

According to one particular embodiment, the said means of determiningthe DC component measure the voltage at the terminals of the secondresistor.

According to one particular embodiment, the said means for determiningthe DC component tap off some of the current at the output of theamplifier towards a current-measuring circuit.

According to one particular embodiment, the current-tapping branchcomprises, in series, a second reference voltage generator and a fourthresistor.

According to one particular embodiment, the said detection means carryout a comparison between the value of the DC component measured by thesaid current measuring circuit and the value of the DC component at theoperating point of the said device.

According to one particular embodiment, the said comparison is carriedout with respect to a range around the value of the DC component at theoperating point of the said device.

According to one particular embodiment, the said determination circuitcomprises a microprocessor connected to a digital-analog converter andto a memory in which is stored the value of the DC component at theoperating point, the said digital-analog converter being linked to theterminals of a resistor connected to the tapping-off branch via anoptocoupler.

The invention employs a detector circuit placed in parallel on thesignal path. Thus a voltage drop possibly caused by a detector in seriesis avoided.

BRIEF DESCRIPTION OF THE DRAWING

Other characteristics and advantages of the invention will emergethrough the description of a particular non-limiting embodimentillustrated by the figures, among which:

FIG. 1a is an electrical diagram of a telephone line,

FIG. 1b is the equivalent of the diagram of FIG. 1a in terms of DCcomponent,

FIG. 2 is a graph of the template of the characteristics of aninstrument, in terms of DC component of voltage and current, as well asthe operating characteristic of a device in accordance with the presentembodiment,

FIG. 3 is an electrical diagram of the device according to the presentembodiment.

DETAILED DESCRIPTION

According to the present embodiment, a digital television decoder isequipped with a modem and connected to the subscriber's STN network, towhich in addition a certain number of other instruments are connected,whether they are modems, conventional telephone instruments or otherapparatus. First of all, before taking the line, the television decoderchecks that the line is not busy. Secondly, during data transmission,the television decoder monitors the line in order to detect a handsetbeing lifted. If the handset is lifted, the transmission of the data isinterrupted and the line freed. Both detections can be carried out byvirtue of the device in accordance with the invention. FIG. 1aillustrates, in a simplified way, the composition of a telephone line ofthe STN network. The installation includes an audio signal generator 1with passband from 300 to 3400 Hz. The internal resistance of thegenerator is symbolized by a 600 ohm resistor 2 placed in series withthe generator.

The resistor 2 and the generator 1 are linked to the subscriber'stwo-wire transmission line (L1, L2) through two DC decoupling capacitors3 and 4. The power supply bridge itself is placed in parallel on the twoconductors: it comprises a voltage generator 5 in series with aninternal resistor 6, and is linked to lines L1 and L2 via an inductor 7and an inductor 8 respectively. The line itself is from 0 to 3.5kilometres long. Having regard to the linear resistance of theconductors, this is equivalent to a resistor lying between 0 and 1100ohms. The various items of subscriber apparatus (telephones T1 and T2,modem and decoder M according to the present example), are connected inparallel on the two conductors.

FIG. 1b is the equivalent diagram in terms of DC component. The voltagegenerator is again present, in series with a variable resistor 9 of 300to 1400 ohms, which incorporates the internal resistance of thegenerator as well as the resistance of the transmission line. Betweenthe two conductors L1 and L2, a line voltage VL is generated. Thisvoltage is, obviously, identical for all the apparatus connected to theconductors. The current flowing in the transmission line is referencedIT, while the current flowing in the modem M is referenced IL.

FIG. 2 illustrates the specified template of the DC characteristics ofapparatus connected to the STN network in France. The voltage, in volts,is given on the abscissa, while the current strength is given inmilliamps on the ordinate. The shaded parts correspond to theunauthorized operating points. Table 1 gives the coordinates(abscissa-ordinate pairs) of the two lines bounding the authorizedoperating region. This region is limited, moreover, by the two loadlines labelled 300 ohms and 1400 ohms respectively.

TABLE 1 Upper limit Lower limit (0; 0)  (0; 0) (3; 30) (8.6; 0)   (4;30) (8.6; 16)  (4; 60) (12.4; 16)   (>4; 60)  (12.4; 12.5) (15.3; 24.5)(15.3; 26)   (>15.3; 26)  

The detection of an item of apparatus is carried out by monitoring thestrength of the DC component of the current flowing in the modem. It istherefore sought to obtain a variation in current strength which iseasily detectable when another item of apparatus connected to thetransmission line is unhooked. Given that nothing can be done to thisother apparatus, it is the operating characteristic of the detectorapparatus, in this instance the modem, which is chosen appropriately.

In the context of the present example, it is the characteristic formedby the lower limit of the template of FIG. 2 which will be used as acharacteristic of a telephone, the unhooking of which it is sought todetect. In fact, this characteristic exhibits a region of zero currentstrength between 0 and 8.6 V.

The curve D, in bold, of FIG. 2 represents the DC voltage and currentcharacteristic of the device in accordance with the present example,while the dashed curve E represents the sum of the characteristic D andthe characteristic of the telephone.

Let us suppose that the operating point of the modem alone is situatedat point X, the intersection between the 1400 ohm load line and thecharacteristic of the modem. The 1400 ohm load line is taken here as anexample because it is the example which takes account of the worstconditions. The same reasoning applies for any other load line. When thetelephone is unhooked, the operating point of the telephone-modemcombination will remain on the same load line, moving to point X′. Fromthe point of view of the modem, the operating point will become X″,situated on the characteristic of the modem and in the vertical from X′,given that the voltage is the same at the terminals of the modem and ofthe telephone.

Let us take the case in which X lies within the zero current strengthrange of the characteristic of the telephone (this is not possible withthe characteristics of the modem illustrated). In the event that thetelephone is taken off the hook, X, X′ and X″ are coincident since thecharacteristic of the modem and the characteristic of themodem-telephone combination are coincident. No detection is thenpossible, since no variation in current or in voltage is felt at themodem.

It is necessary therefore for the operating point X to be situatedoutside of the zero current strength range, that is to say beyond 8.6 Vin the present example.

The template limits the DC component of current to 60 mA, which impliesa bend in the characteristic of the modem. In order to maximize thecurrent variations introduced by unhooking the telephone, a slight bendis chosen, and thus a slope such that the characteristic of the modemrejoins the 300 ohm load line at about 60 mA of current strength. Giventhat it is the variation in current between the operating point of themodem before unhooking the telephone and the operating point of themodem after unhooking the telephone which is detected, a steep slope ofthe characteristic accentuates the variation in direct current withinthe modem.

FIG. 3 is an electrical diagram of the device in accordance with thepresent embodiment. The device includes a circuit 10 for protectionagainst surges, consisting of a resistor RP in series with one of theconductors L1 or L2, as well as a resistor 11 varying as a function ofthe applied voltage and placed in parallel on the two conductors, afterRP. It will be remembered that the input voltage of the device is VL.

A MOS relay with a resistance of RO in the closed state is placed inseries with RP, after the resistor 11. The two conductors feed arectifier bridge consisting of four diodes Dl to D4. The output of theMOS relay 12 is connected between the diodes D1 and D2, the conductor L2being connected between the diodes D3 and D4. The connections betweenthe diodes D2 and D3, and D1 and D4 respectively, represent the“positive” P and the “neutral” N outputs of the rectifier.

The outputs of the rectifier bridge are connected to an audio processingcircuit 13. This circuit 13 includes a capacitor 14 for decoupling theDC component, linked in series between the point P and the primary of atransformer. The primary is also linked to the point N. The load on thesecondary of the transformer is represented by a load impedance 16.

The collectors of two transistors T1 and T2 mounted as a Darlington pair(18) are also connected to the point P. The base of transistor T1 islinked to the midpoint of a resistor bridge formed by a resistor R1connected to the point P and a resistor R3 connected to the point N viaa diode D5 in series with a Zener diode DZ. The Darlington pair may bereplaced by another type of amplifier with sufficient gain.

The output of the Darlington pair, in this instance the emitter oftransistor T2, is linked to the point N on the one hand via a resistorR2, and on the other hand via two diodes D6 and D7, a resistor R4 andthe emitter diode of an optocoupler 19 in series. The midpoint of theresistor bridge R1 and R3 is also connected to the point N via acapacitor 17. The role of this capacitor is to filter the frequenciescorresponding to the audio signals, in such a way as to avoid thissignal disturbing the current in R2.

One function of the device is to create a DC characteristic meeting thecriteria defined above. According to the present example, thecharacteristic is composed substantially of two straight line segments Aand B which join together at a bend C.

Two reference voltages are used by the circuit. The first one, Vref1, isdetermined by the diode D5 and the Zener diode. Vref1 is chosen to lieat about 8.6 V with respect to the voltage reference of VL. The diode D5has a temperature-dependent behaviour similar to that of the Darlingtonpair and increases the invariability of the temperature-dependentcharacteristics of the circuit. The second reference, Vref2, isdetermined by the two diodes D6 and D7. It is employed to detect theunhooking.

At first, the situation is that the voltage of 8.6 V is not reached byVL: the diode D5 and the Zener diode do not conduct. In this case, itcan be shown that the slope A of the characteristic is substantiallyequal to Rp+R0+R2, the influence of R1 being made negligible by the highgain of the Darlington pair. Taking Rp=10 ohm, R0=30 ohm and R2=270 ohma slope A is obtained of 1/310 ohm⁻¹. The slope A can thus easily beadjusted by altering the resistor R2.

Two diodes of the rectifier bridge, as well as the two emitter-basejunctions of T1 and T2 introduce an offset voltage Vs in the circuit.This voltage explains why the straight-line segment of the slope A doesnot start from the origin of the graph of FIG. 2, but is slightlyoffset. If each diode and each junction is considered to represent avoltage of 0.6 V, then Vs is substantially equal to 2.4 V.

When VL exceeds 8.6 V, D5 and the Zener diode conduct. In this case itcan be shown that the slope of the segment B is given approximately bythe relation: R2×(R1+R3)/R3. Taking the numerical values of R1=12 kohmand R3=2.7 kohm, a slope of 1/1470 ohm⁻¹ is obtained.

The current in the emitter of T2 is substantially equal in absolutevalue to the direct current in RP. The voltage at the terminals of theresistor R2 gives an adequate image of this current. The diodes D6 andD7 and the resistor R4 tap off a part of this current to the optocoupler19. The reference voltage created by D6 and D7, as well as the value ofR4 (2.7 kohm according to the present example) calibrate the currentpassing through the diode of the optocoupler 19. The phototransistor ofthis optocoupler is linked to a circuit 20 for determining the value ofthe DC component of the current. The value thus obtained is compared toa range of values situated around the operating point X of FIG. 2. Whenthe measured value of the current lies outside of this region, it isdetected that there is an instrument in operation connected to thetelephone line.

The value of the current strength at the operating point X is stored ina memory of the circuit 20. The latter also comprises a microprocessoras well as an analog-digital converter carrying out the conversion ofthe information supplied by the optocoupler into data which can beinterpreted by the microprocessor. According to the present embodiment,a variation of the order of magnitude of a few mA around this valuedetermines the abovementioned range.

According to one particular embodiment, the said value of the strengthof the current at the operating point is reprogrammed automatically bythe circuit 20 from time to time, for example upon each successful lineacquisition. This variant makes it possible to take account of theageing of certain components, particularly the optocoupler.

The detection circuit is employed on various occasions:

In the first place, when the apparatus including the modem has to takethe line, the modem is set in operation for a brief time, the time tostabilize the current within the circuit and to determine the value ofthe DC component of the current. In this way, it is possible to checkwhether the line is busy.

Secondly, the determination of the DC component is carried outperiodically when a data transmission is in progress. If it is detectedthat another apparatus is in operation during this transmission, thetransmission is interrupted to give priority to the detected apparatus.The transmission is resumed after a waiting period.

The periodicity of the determination of the DC component is intended toavoid excessive use of the microprocessor of the circuit 20, which canbe used for other tasks.

According to a variant embodiment, the circuit 20 comprises a circuitfor tapping off the current supplied by the optocoupler. A variation inthe current then triggers the stopping of a data transmission by causingan interrupt within a microprocessor controlling the modem. In thiscase, the device is only used for detection of the unhooking of anotherapparatus during a communication.

It should be noted that the closer the characteristic of the modem is tothe template, the more insensitive it needs to be to temperaturevariations. According to one particular embodiment, in order to avoidgoing outside the template, a safety distance is provided between thelimits of the template and the characteristic of the modem.

The embodiment described relates to an apparatus including a modem andtransmitting modulated data via the STN network. It is quite obviousthat the invention is not limited to this example, any apparatus linkedto the STN network or to a network having similar properties may bemodified by the person skilled in the art in order to implement theinvention.

According to the example given, the operating point, in DC terms, of themodem is chosen to lie within a region of the template where the DCcomponent of the current cannot be zero, this being done to avoidnon-detection of an apparatus effectively exhibiting a zero DCcomponent. According to a variant embodiment, those regions of thetemplate are also avoided where the DC component of the current ofapparatus other than the modem may be low enough not to induce a currentvariation detectable by the modem.

The template in terms of DC components of FIG. 2 is the template validin France. Nevertheles , the invention is not limited to this particulartemplate.

What is claimed is:
 1. Communications device connected in parallel withat least one item of communications apparatus on a transmission line,the said device and the said apparatus conforming to a given template interms of DC components, the said device being characterized in that itcomprises: means setting the operating point of the said device alone,in terms of DC component, outside regions of the said template where theDC component of the current may be zero, means of determining the DCcomponent of the current flowing in the said device, means of detectinga variation in the said DC current component.
 2. Communications deviceconnected in parallel with at least one item of communications apparatuson a transmission line, the said device and the said apparatusconforming to a given template in terms of DC components, the saiddevice being characterized in that it comprises: means setting theoperating point of the said device alone. in terms of DC component,outside regions of the said template where the DC component of thecurrent may be zero; means of determining the DC component of thecurrent flowingq in the said device; means of detecting a variation inthe said DC current component; and wherein: the said means of settingthe operating point comprise means of generating a characteristic, interms of DC components, such that it possesses intersections with thepossible load characteristics of the transmission line which aresituated in regions of the said template where the DC current componentcannot be zero.
 3. Device according to claim 2, characterized in thatthe said characteristic in terms of DC components comprises a point ofintersection with the 1400 Ohm load characteristic, the said point beingsituated beyond 8.6 V.
 4. Device according to claim 2 characterized inthat, with the said template including a maximum current limit, thecharacteristic of the said device includes a maximum slope in the regionbounded by the load characteristics and the said maximum limit, the saidcharacteristic having a point which is common with each possible loadcharacteristic.
 5. Device according to claim 1 characterized in that thesaid means of detecting a variation in the said DC component comprise acircuit forming the derivative of the signal corresponding to the saiddefined DC component.
 6. Device according to claim 1, characterized inthat the said means of detecting a variation in the said DC componentcomprises means of comparing the said defined DC component and the DCcomponent at the operating point of the said device alone.
 7. Deviceaccording to claim 1, characterized in that it comprises a bridgerectifying the voltage of the line interposed between the transmissionline and an audio signal processing circuit.
 8. Communications deviceconnected in parallel with at least one item of communications apparatuson a transmission line. the said device and the said apparatusconforming to a given template in terms of DC components, the saiddevice being characterized in that it comprises: means setting theoperating point of the said device alone, in terms of DC component,outside regions of the said template where the DC component of thecurrent may be zero; means of determining the DC component of thecurrent flowing in the said device; means of detecting a variation inthe said DC current component; a bridge rectifying the voltage of theline interposed between the transmission line and an audio signalprocessing circuit; and wherein: the positive output of the rectifierbridge being connected to the collectors of a first and secondtransistor mounted as an amplifier, the said positive output beinglinked to the base of the first transistor through a first resistor, thesaid amplifier feeds into a second resistor linked to the neutral outputof the said rectifier bridge.
 9. Device according to claim 6,characterized in that it further comprises a capacitor linking the baseof the first transistor to the neutral output of the rectifier bridge.10. Device according to claim 8, characterized in that it furthercomprises, in series between the input of the amplifier and the neutraloutput of the rectifier bridge, a third resistor, and on/off means and afirst reference voltage generator, the said on/off means being closedwhen the line voltage exceeds a given value.
 11. Device according toclaim 10, characterized in that the said on/off means and the saidvoltage source comprises a Zener diode.
 12. Device according to claim 8,characterized in that the said means of determining the DC componentmeasure the voltage at the terminal s of the second resistor.
 13. Deviceaccording to claim 8, characterized in that the said means fordetermining the DC component tap off some of the current at the outputof the amplifier towards a current-measuring circuit.
 14. Deviceaccording to claim 13, characterized in that the current-tapping branchcomprises, in series, a second reference voltage generator and a fourthresistor.
 15. Device according to claim 13, characterized in that thesaid detection means carry out a comparison between the value of the DCcomponent measured by the said current measuring circuit and the valueof the DC component at the operating point of the said device. 16.Device according to claim 15, characterized in that the said comparisonis carried output with respect to a range around the value of the DCcomponent at the operating point of the said device.
 17. Deviceaccording to claim 16, characterized in that the said means ofdetermining the DC component of the current flowing in the said devicecomprises a microprocessor connected to a digital-analog converter andto a memory in which is stored the value of the DC component at theoperating point, the said digital-analog converter being linked to theterminals of a resistor connected to the tapping-off branch via anoptocoupler.